Attempts have been made previously to directly couple switching semiconductor devices to the grid in order to maintain phase synchronisation and simplify system design. One of the key problems is how to rapidly remove gate charge and at the same time minimise power loss in the driving circuit. If charge is not removed rapidly from the gate terminal a shoot-through problem results and a ground fault develops on the grid. One possible solution is to use small ohmic resistors. However, the use of small ohmic resistors to form a potential dividing circuit often results in huge losses in the resistors and therefore reduced lifetime and reliability for the system.
Also, many micropower generation systems, such as those in the home, typically comprise one or more of a number of solar cells (e.g. on the roof), wind turbines, combined heat and power systems and other like systems. The micropower generators generate electricity for the home, and the power is converted into useable voltage and current suitable for the home, for example 240V at 50 Hz or 110V at 60 Hz. However, more power than is actually required by the home is sometimes generated. If the micropower generation system were connected to the AC grid, from which power is normally drawn in conventional homes, this surplus power could be sent back to the AC grid.
However, there exists a need for a system of synchronising the power generated by the converters to the power on the grid. Inverters are often used to generate an AC output from a DC input. The inverters are generally located within the proximity of the power source (solar cells, wind turbine etc). The point at which the inverter is connected to the AC grid mains is often remote from its physical location.
Inverters connected to the grid remotely may experience a phase shifted line voltage due to line impedance and therefore transfer an increased amount of reactive power in the network. The increase in reactive power implies minimised system efficiency.
There is therefore a need to enable the synchronisation of inverters to the same line voltage regardless of the line impedance between power switchboard and the point of connection of the inverter.
We describe techniques to address the above problems.